The present invention relates to a new and improved construction of headbox for a papermaking machine.
Generally speaking, the headbox for a papermaking machine of the present development is of the type comprising an infeed channel for the stock suspension at which merges a nozzle channel bounded by two substantially rigid lips or lip members. These lip members are oriented towards a substantially band-shaped dewatering surface guided over a rotating water pervious or foraminous roll or cylinder. The first lip member possesses an end intended to approach the dewatering surface and the second lip member possesses a rigid lip portion which protrudes past the end of the first lip member in the flow direction of the stock suspension. This rigid lip portion is provided with a guide surface which is concave with respect to the stock suspension. This concave guide surface covers a portion of the dewatering surface neighboring the end of the first lip member in the downstream direction thereof and by means of such portion delimits a sheet forming chamber or space which converges in the direction of movement of the dewatering surface.
According to a heretofore known apparatus of this type the first lip member and the lip portion of the second lip member are guided at a portion of a wire or sieve forming the dewatering surface. This wire portion extends over part of the circumference of a suction breast roll. The first lip member and the lip portion of the second lip member each possess an inner surface which is smooth over its entire length. Significant in this regard is the headbox construction disclosed in U.S. Pat. No. 4,133,713, granted Jan. 9, 1979. With this type of design of headbox there can be observed a reduction of the flow velocity which is caused by wall friction. This reduced flow velocity appears at the outer marginal region of the flow extending along the second lip member. Consequently, the stock fibers dispersed in the stock suspension, particularly when encountering higher flow or machine velocities, tend to increasingly orient in the flow direction. As a result, the tear or breaking strength of the paper which is to be fabricated can be impaired in a direction transversely with respect to the flow direction and the travel direction of the dewatering surface, respectively.
According to another state-of-the-art construction of a similar type, as disclosed in U.S. Pat. No. 4,125,429, granted Nov. 14, 1978, the second lip member is provided with a flexible wall portion which covers the sheet forming chamber or space. This flexible wall portion snugly adapts itself to the flow of the stock suspension. From this prior art reference it is also known, but in a different connection, i.e. for the purpose of preventing the clumping together or agglomeration of the stock fibers, to equip the flexible wall portion with turbulence generators. However, these turbulence generators cause a pronounced directional change in the marginal flow, and thus, can lead to the formation of lengthwise turbulence pairs which appear in the form of lengthwise streaks in the paper which is fabricated. Also with this prior art construction there can arise the previously described drawbacks caused by the wall friction at the marginal flow extending along the flexible wall portion.
Other constructions of headbox are known to the art, such as for instance typified by U.S. Pat. No. 4,280,870, granted July 28, 1981 which use step-shaped widened portions within the guide channel for the stock suspension and an adjustable lip for regulating the size of the outlet opening or slice.